With time, reservoirs of compressed gas systems accumulate by condensation or separation from the contained compressed gas, mainly liquid matter which in diesel locomotive or other vehicular compressed air systems, usually contains some oil but is predominantly water. If the accumulated liquid is not periodically drained from a reservoir of such a system, it and any entrained solid particles, can corrode, clog or otherwise damage mechanisms operated by the compressed air or other gas from the reservoir. It therefore is customary to fit such a reservoir with a drain valve for periodically and preferably automatically draining the accumulated matter. Exemplary of such drain valves is that of Frantz U.S. Pat. No. 3,262,464, in which a valve member in the form of a differential piston normally held closed by a return spring, is opened for draining accumulated matter from a reservoir on each idling cycle of an associated compressor by actuating compressed gas supplied by the compressor's unloader or governor line. While that patent also suggests that the drain valve might be operated automatically by a timer-controlled solenoid, if, as in that patent, the only force normally holding the valve closed is that of the return spring and if, as in a diesel locomotive, the maximum reservoir pressure is about 140 p.s.i.g., the required opening force would be beyond the capacity of the usual single coil solenoid. Other problems confronted in drain valves for compressed gas reservoirs are freezing if the valve is exposed at times to ambient freezing temperatures and excessive loss of reservoir pressure, if, as heretofore, the valve is opened automatically at predetermined intervals without regard to the presence of any accumulated matter. It is to these and other problems confronted in adapting a drain valve for a compressed gas reservoir to solenoid actuation that the present invention is particularly directed.